residual valves are supposed to be used only if the master cylinder is lower than the caliper or wheel cylinder. think of a hot rod with the master cylinder below the floor boards. this is to prevent drainback into the master cylinder. that is why speedway says to use the 2# for the front and the 10# for the rear. some have said they have used these in systems with the master cylinder on the fire wall, but i believe there may have been other issues with their systems and these valves have just been a band-aid fix.